This invention relates to a dual-circuit pressure control valve for brake systems including two control pistons arranged in parallel side by side and each assigned to a different brake circuit, and a common unguided preloading device, for instance, with a helical spring acting on the pistons through a distribution device, pressing each of the pistons at rest against a stop, which in the event of a pressure failure in one brake circuit increases the preload on the control piston of the other brake circuit.
In a known dual-circuit pressure control valve of this type, such as shown in FIGS. 5 to 7 of German Patent DE-OS No. 27 48 699, the distribution device includes a mushroom-like part, which projects with its stem into the somewhat larger blind bore between the control pistons, abuts with the bottom of the head on the end surfaces of the control pistons and is loaded directly by a helical spring on the top of its head. The stem is somewhat thickened at the lower end, so that it is guided in the blind bore. Each of the control pistons is a component of a different pressure control valve, each of which is inserted in a different one of two independent brake circuits, each leading from a different outlet pressure chamber of a tandem master cylinder, or an equivalent pressure generator, to a different one of the rear axle wheel brake cylinders of a cross-connected dual brake system. The control function can be different. The control valves can, for example, operate as a stop valve, which keeps the outlet pressure constant with rising inlet pressure after attainment of the change-over point, or as a pressure reducing valve, which makes the outlet pressure increase slower upon rising inlet pressure after the change-over point has been reached.
Such a distribution device sees to it that in normal operation half of the pressure of the helical spring is effective on each control piston and that both pressure control valves have the same change-over point. To accomplish this the stem of the mushroom shifts axially in the blind bore. If, however, one brake circuit fails, the whole pressure of the spring acts on the control piston of the other brake circuit, resulting in the change-over point there being increased to double the value and a correspondingly higher brake effect being achieved in the pertinent wheel cylinder. This is, however, possible only to an imperfect degree. As long as the mushroom head is tilted, only half of the force of the spring is acting on the control piston still in operation. Only in the event that the tilting movement is limited by abutment of the stem on both sides of the blind bore will the total force of the spring be in a position to act on the operating control piston when the mushroom head is shifted axially. This axial shift, however, takes place only after overcoming considerable sliding forces, so that no specific power ratio can be given for the determination of the change-over point. Additionally, the tilting angle of the mushroom head cannot be kept very small due to the comparatively short stem, whereupon the distance to be covered by the control piston with only half the spring load is a long one. A further disadvantage is that due to the arrangement of the blind bore between the two control pistons, a predetermined and desired housing width cannot be maintained.
It is furthermore common to use a guided preloading device, such as disclosed in FIGS. 1 to 4 and 8 of German Patent DE-OS No. 27 48 699. For this purpose a cup-shaped element is provided, which is loaded by a helical spring. The cup-shaped element is guided in the housing by its circumferential surface, has a mechanism to prevent twisting thereof and has a means for supporting a distribution lever on its bottom. The adjacent ends of the two control pistons are in contact with the ends of the distribution lever under normal operating conditions. If one brake circuit fails, the control piston of the other brake circuit can be operatively connected, after a short movement, with the bottom surface of the cup-shaped element, thus, having the whole spring force acting on it. Such a preloading device, however, entails considerable cost and material. Also, the preloading device is to a great extent loaded eccentrically under emergency operation conditions and, therefore, will also be subject to certain sliding forces.